The present invention relates to the subsea anchoring of offshore jacket structures and, more particularly, to the grouting of jacket legs to a seabed.
Offshore structures of the type utilized in the drilling of oil or gas wells may comprise a jacket which is to be installed upon the seabed and a platform which is to be supported on the jacket above the water surface. Generally, the jacket is fabricated at a land-based facility, then towed on its side in a floating condition to an offshore work site and thereafter erected within the water and lowered onto the seabed. The jacket may comprise a plurality of legs or sleeves which rest on the waterbed and project upwardly to the water surface to carry a platform. The jacket sleeves are anchored to the seabed by means of piles which are driven through and beyond the pile sleeves and into the seabed. Apparatus of this type is disclosed, for example, in U.S. Pat. No. 3,823,564, issued to Crout et al on July 16, 1974 and assigned to the assignee of this invention. The disclosure of that patent is incorporated herein by reference as if set forth at length.
To permanently secure the piles in place within the sleeves, grout is introduced into the annulus formed between the outer face of each pile and an inner wall of its associated sleeve. The grout extends essentially the entire height of the annulus and somewhat below the mudline of the seabed.
In some instances the jacket is anchored solely by piles driven through the main jacket legs or sleeves, while in other instances secondary pile sleeves, or skirt sleeves, are utilized which are of relatively short height and are attached by framing to lower ends of the main sleeves. The secondary pile sleeves can be distributed around the baase of the jacket or arranged in clusters.
Once anchored, the jacket is subject to wave and current action which imposes considerable forces on the main jacket sleeves. Axial forces are transmitted from the main sleeves to the main piles via the grout connection. In cases where skirt piles are employed, it is intended that an appreciable portion of such loads be transmitted to the skirt piles via framing interconnecting the main and skirt sleeves to distribute the loads more evenly. It has been found in practice, however, that such loading may not be distributed in the most effective manner due to a certain degree of inherent flexibility in the framing which tends to restrict to transmission of forces to the secondary sleeves. As a result, the main piles are required to resist a greater proportion of the loading than is desired.
It has also been found that, whether or not skirt piles are employed, there may occur an uneven distribution of loads among the main jacket sleeves. That is, the piles of the main jacket sleeves located farther from the central axis of the jacket base are likely to bear a greater proportion of the loading than the piles of main jacket sleeves located closer to such central axis.
From the foregoing, it will be appreciated that prudence has heretofore dictated that at least some of the main sleeves and piles have had to be made larger than desired in order to compensate for the uneven distribution of loading among the various anchoring piles. This has resulted in added costs and other complications.
It is, therefore, an object of the present invention to minimize or obviate problems of the type discussed above.
It is another object of the present invention to provide novel anchoring arrangements and anchoring methods for an offshore jacket.
It is a further object of the invention to enable the distribution of forces in an offshore jacket to be more precisely controlled.
It is an additional object of the invention to create a more uniform distribution of loading among the anchoring piles of an offshore jacket.
It is another object of the invention to provide a jacket anchoring arrangement wherein the lower level of grout in at least some of the pile sleeves terminates at a level above the lower level of grout in other sleeves located closer to the central axis so that the axial spring value of the former piles is decreased whereby the distribution of forces among the piles is more uniform.
It is a further object of the invention to provide a jacket anchoring arrangement wherein skirt pile sleeves are connected to main pile sleeves and wherein the lower level of grout in at least some of the main pile sleeves terminates above the seabed to provide greater flexibility in the framing and thereby transfer a greater amount of loading to the secondary pile sleeves than would otherwise occur.